The present invention relates to a method for acoustically surveying the surface contours of the bottom of a body of water, particularly the bottom of the ocean. More particularly, the present invention relates to a method for acoustically surveying the surface contours of the bottom of a body of water, particularly the bottom of the ocean, with the use of an echo sounder installed on a ship and including a transmitting and a receiving device wherein the transmitting device emits sound pulses over a narrow (when seen in the direction of the ship's heading) transmitter target strip extending on the bottom of the body of water essentially perpendicularly below the center of the transmitting antenna and transversely to the ship's heading, wherein the transmitter target strip is covered, by means of the receiving device, by a fan which extends, in the ship's heading, in the form of a series of narrow receiving strips forming a row transversely to the ship's heading with one receiving strip being disposed essentially perpendicularly below the center of the receiving antenna, and wherein the echos produced at the bottom of the body of water in the transmitter target strip are received separately from the individual receiving strips, with the travel times of the received echoes being measured and depth values being determined therefrom to produce a measured contour of the transmitter target strip extending transversely to the ship's heading, in spatial association with the respectively determined point of origin of the echoes as determined by the momentary position of the transmitter target strip and the respective receiving strip.
The contours measured according to this process of the individual transmitter target strips, when aligned in the direction of the heading of the surveying ship, will provide a surface contour representation that corresponds to the actual conditions of the scanned bottom of a body of water only if the speed of sound can be considered to be essentially constant over the entire ranging area. However, this can generally not be assumed to be the case. Rather, it is characteristic for ocean water that thin, changing layers are formed which separate layers of water of different density or speed of sound. As a result of climatic or oceanic phenomena, such as, for example, heating or cooling in the course of a year or day, churning due to heavy seas, formation of internal waves in a period of time from minutes to hours, and/or confluence of masses of water from different origins, the layers having different speeds of sound are constantly changing. The consequences are distorted surface contours.
In a known method of the above-mentioned type (U.S. Pat. No. 3,296,579, issued Jan. 3, 1967) compensation for the contour distortions is provided in that a computer receives so-called bathythermogram data in order to calculate the measured contours from the echo travel time and the receiving direction. Under consideration of these data, the computer calculates the corresponding spatial, horizontal and vertical coordinates of the actual origin of the echo. The barhythermogram data are picked up from bathythermograph recordings. Since the layers of water within the sea area being surveyed may change due to the above-mentioned phenomena by the minute or the hour as well as due to the increased distances traveled by the surveying ship, it is necessary for accurate surveying work to produce such bathythermograms continuously or at very short time intervals in the area where the coutour measurements are taking place. However, recording a bathythermogram is a complicated and time consuming procedure which reduces the operating speed of the surveying ship to a barely acceptable degree. Therefore, one usually is satisfied with producing such bathythermograms at greater spatial or time intervals, which then is done at the cost of the accuracy of the surface contours being measured.